I recently thought I had a belt failure on my daughters beetle, she didn’t but thats another story.
Belt failure is usually for 1 of 2 reasons. You left it on to long without changing it and one of the moving parts seized breaking the belt.
Or it was installed incorrectly. It sounds like the engine you’re looking at with the marks is a “mark and pray” job where you mark up the cam,, crank, pump and whatever else, pull the old belt off and pray that the new belt goes back on correctly, instead of using the proper tools.
Rotating the tensioner in a counterclockwise direction will kill your tensioner yet some do exactly that. Not loosening the cam or pump sprocket during tensioning can result in an incorrectly tensioned belt. A properly installed belt should not jump time, an improperly installed tensioner can cause the belt to jump time.
If the belt slipped or broke, the person doing the work needs to pull the head and determine if valve and piston contact has been made. I assume the engine you mentioned is not yours.
Did you do the adjustments on yours yet?